Fiber laser, spontaneous emission light source and optical fiber amplifier

ABSTRACT

New fiber lasers, spontaneous emission sources, and optical fiber amplifiers are provided. Their conventional counterparts, which have a fiber doped with thulium (Tm) ions and excited by 0.67 μm or 0.8 μm pumping light, have a problem in that their characteristics are deteriorated with the elapse of time. The new fiber lasers, spontaneous emission sources, and optical fiber amplifiers use 1.2 μm light as pumping light. Alternatively, they use a pumping source for exciting the thulium from the lowest energy level  3 H 6  to  3 H 5  excitation level. As a more preferable configuration, they improve the emission efficiency at 2.3 μm band by disclosing Tm-doped host glass.

TECHNICAL FIELD

The present invention relates to a fiber laser, spontaneous emissionlight source, and optical fiber amplifier, and particularly to the fiberlaser, spontaneous emission light source, and optical fiber amplifierthat operate near a 2 μm band with using as the gain medium an opticalfiber having a core or a cladding doped with a rare-earth element havinga laser transition level.

BACKGROUND ART

FIG. 1 is an energy level diagram of a thulium ion (see, non-patentdocument 1). In FIG. 1, energy values are shown on the right side ofindividual energy levels, and names of the individual levels are shownon the left side of the individual energy levels. Numerals added toarrows indicate wavelengths of light absorbed (corresponding to upwardarrows (not shown in FIG. 1)) or emitted (corresponding to downwardarrows in FIG. 1) when transitions of the individual arrows occur. Here,the unit of the energy is represented by 1/cm (corresponding to Kayserin terms of spectroscopy) based on the unit of the wave number, and thename of the energy levels are based on the Russell-Saunders notationalsystem. In addition, alphabetical capitals represent a compound orbitangular momentum, superscript index digits added to them represent themultiplicity of the spectral term based on electronic total spin angularmomentum, and subscript index digits added to them represent the totalangular momentum. Here, ³H₆ level is the level having an expanded widthbecause of the segmentation of degeneration levels by the Starke effectcaused by crystal electric field.

As for a fiber having its core doped with thulium (Tm), applications tofiber lasers, spontaneous emission sources, or optical fiber amplifiershave been studied employing the following bands of the thulium ion inFIG. 1:

-   -   1.9 μm band using ³H₄→³H₆ transition (which represents the        transition of the thulium ion energy from the ³H₄ level to the        ³H₆ level. This notational system will be used from now on);    -   2.3 μm band using ³F₄→³H₅ transition;    -   0.82 μm band using ³F₄→³H₆ transition; and    -   1.48 μm band using ³F₄→³H₄ transition.        Incidentally, to implement the fiber lasers, spontaneous        emission sources, or optical fiber amplifiers at a high        efficiency between the transitions above-mentioned, fluoride        fibers are used as fibers to which Tm (thulium) is added. Among        the Tm-doped fluoride fibers, the 2.3 μm band, in particular, is        difficult to oscillate by semiconductor lasers, has hidden        potential to become a huge business, and attracts great        attention as a light source for noninvasive blood glucose level        sensing which many foreign and domestic medical inspection        instrument developers compete fiercely to develop.

Up to now, the following have been reported:

-   (1) Laser oscillation at 0.82 μm band, 1.48 μm band, 1.9 μm band and    2.35 μm band implemented by applying 0.67 μm band excitation    (excitation of the thulium ions at the ³H₆ level to the ³F₃ level)    to the Tm-doped fluoride fiber (see, non-patent document 1);-   (2) Laser oscillation at 2.35 μm band implemented by applying 0.8 μm    band excitation (excitation of the thulium ions from the ³H₆ level    to the ³F₄ level) to the Tm-doped fluoride fiber (see, non-patent    document 2 or patent document 1);-   (3) Laser oscillation at 0.82 μm band, 1.48 μm band, 1.9 μm band and    2.35 μm band implemented by applying 0.8 (0.79) μm band excitation    to the Tm-doped fluoride fiber (see, patent document 1);-   (4) Laser oscillation and an optical fiber amplifier at 1.9 μm band    implemented by applying 1.55-1.75 μm band excitation, excitation of    the thulium ions from the ³H₆level to the ³H₄ level, to the Tm-doped    fluoride fiber (see, patent document 2); and-   (5) Laser oscillation and an optical fiber amplifier at 1.48 μm band    implemented by applying 1.06 μm band excitation to the Tm-doped    fluoride fiber (see, patent document 2).

The 2.3 μm band fiber lasers have already been developed as described inthe foregoing reports (1), (2) and (3).

Patent Document 1: Japanese Patent Application Laid-open No. 3-293788(1991);

Patent Document 2: Japanese Patent Application Laid-open No. 6-283798(1994);

Non-Patent Document 1: J. Y. Allain et al., “Tunable CW lasing around0.82, 1.48, 1.88 and 2.35 μm in Thulium-doped fluorozirconate fiber”Electron. Lett., Vol. 25, No. 24, pp. 1660-1662, 1989;

Non-Patent Document 2: L. Esterowitz et al., “Pulsed laser emission at2.3 μm in a Thulium-doped florozirconate fiber”, Electron. Lett., Vol.24, No. 17, p. 1104, 1988;

Non-Patent Document 3: A. Taniguchi, et al., “1212-nm pumping of 2 μmTm-Ho-codoped silica fiber laser”, Appl. Phys. Lett., Vol. 81, No. 20,pp. 3723-3725, 2002; and

Non-Patent Document 4: P. R. Barber, et al., “Infrared-inducedphotodarkening in Tm-doped fluoride fiber”, Opt. Lett., Vol. 20 (21),pp. 2195-2197, 1995.

DISCLOSURE OF THE INVENTION Problems to be Solved by the PresentInvention

As for the 0.67 μm or 0.8 μm band excitation, however, only those thatuse Tm-doped fluoride fibers as their active medium are reported, and noreports have been presented about Tm-doped fibers that use other glasshost (preform). In other words, it has not been disclosed up to now whatkind of glass host fibers are suitable for the 2.3 μm band operationlaser application.

Furthermore, launching high-intensity light with a wavelength equal toor less than 1.05 μm into a Tm-doped fluoride fiber brings about aphenomenon that causes photo darkening (see, non-patent document 4) thatincreases the loss of the fluoride fiber itself. FIG. 2 illustrates aloss spectrum (solid curves) before launching 1.047 μm band Nd-YLF laserbeam of 500 mW into a Tm-doped fluoride fiber (with an additive densityof 2000 wt. ppm, a fiber length of 20 m, and a relative refractive indexdifference of 3.7%), and a loss spectrum (broken curves) 56 hours afterthe launch of the laser beam. The loss increase of FIG. 2 is due todefects produced in the glass of the fluoride fiber by launching thelaser beam into the fiber. The phenomenon becomes more conspicuous asthe incident wavelength becomes short. Accordingly, considering the 2.3μm band operation laser applications employing the Tm-doped fluoridefibers utilizing the 0.67 μm or 0.8 μm excitation, there arises aproblem in the reliability in that the oscillation efficiency decreaseswith time, and finally the laser oscillation cannot be achieved.Accordingly, using such a conventional light source for the applicationto a noninvasive blood glucose evaluation equipment or the like cannotmake a reliable, practical light source.

Incidentally, 1.9 μm band laser oscillation by a Tm³⁺-Ho³⁺-codopedfluoride fiber using the 1.2 μm band excitation has been reported (see,non-patent document 3). However, it does not utilize the lasertransition of Tm³⁺ from the ³H₄ to ³H₅ level, and hence has nothing todo with the light emission at the 2.3 μm band.

Means for Solving the Problems

The present invention is implemented to solve the foregoing problems.Therefore, main objects of the present invention are:

1) Disclosing glass hosts to be doped with Tm required for the 2.3 μmband operation, and certainly implementing a fiber laser, an amplifiedspontaneous emission (ASE) source, and a fiber amplifier positivelywhich operate in that band; and

2) Implementing high reliability without degradation in the fibercharacteristics due to the photo darkening.

To accomplish the foregoing objects, the present invention ischaracterized by the following two features.

1) As the glass hosts to be doped with Tm required for the 2.3 μm bandoperation, glass whose nonradiative relaxation rate due to themulti-phonon relaxation is less than that of silica glass is used.

2) As the excitation wavelength launched into the Tm-doped fiber foreliminating the degradation in the fiber characteristics due to thephoto darkening, 1.2 μm band is used.

Because of these features, the present invention can carry out theapplication to practical equipments such as the noninvasive bloodglucose evaluation equipment.

Advantageous Results of the Invention

The present invention can offer the following advantages because of theforegoing features:

1) It can positively implement the fiber laser, ASE source (spontaneousemission source) and optical fiber amplifier operating at the 2.3 μmband because it uses the glass whose nonradiative relaxation rate due tothe multi-phonon relaxation is less than the nonradiative relaxationrate of the silica glass as the glass host to be doped with Tm; and

2) It can implement the highly reliable, practical fiber laser, ASEsource and optical fiber amplifier operating at the 2.3 μm band withlittle degradation in the fiber characteristics due to the photodarkening because it utilizes the 1.2 μm band (1.2 μm band excitation)as the excitation wavelength to be launched into the Tm-doped fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy level diagram of thulium ions;

FIG. 2 is a graph illustrating photo darkening of a Tm-doped fluoridefiber;

FIG. 3 is a graph illustrating characteristics of the nonradiativerelaxation rates of various types of glass;

FIG. 4 is a graph illustrating 2.3 μm band spontaneous emission spectrabased on 0.67 μm band excitation in accordance with the presentinvention;

FIG. 5 is a graph illustrating 2.3 μm band spontaneous emission spectrabased on 0.8 μm band excitation in accordance with the presentinvention;

FIG. 6 is a graph illustrating 2.3 μm band spontaneous emission spectrabased on 0.8 μm band excitation and 0.67 μm band excitation inaccordance with the present invention;

FIG. 7 is a diagram illustrating spontaneous emission spectra of a 1.2μm band excitation Tm³⁺-doped fluoride fiber and Tm³⁺-doped telluritefiber in accordance with the present invention;

FIG. 8 is a graph illustrating spontaneous emission spectra of a 1.2 μmband excitation Tm³⁺-doped germanate hydroxide glass fiber, Tm³⁺-dopedchalcogenide glass fiber, Tm³⁺-doped bismuth based glass fiber andTm³⁺-doped fluorophosphate glass fiber in accordance with the presentinvention;

FIG. 9 is a graph illustrating efficacy of the 1.2 μm band excitation inaccordance with the present invention;

FIG. 10 is a schematic diagram showing a configuration of a fiber laserof a first embodiment in accordance with the present invention;

FIG. 11A is a graph illustrating characteristics of a 2.3 μm band-passfilter in the first embodiment in accordance with the present invention;

FIG. 11B is a graph illustrating characteristics of a 1.8 μm band-passfilter in the first embodiment in accordance with the present invention;

FIG. 12 is a graph illustrating output characteristics of the 2.3 μmband fiber laser of the first embodiment in accordance with the presentinvention;

FIG. 13 is a schematic diagram showing a configuration of an opticalfiber amplifier of a second embodiment in accordance with the presentinvention; and

FIG. 14 is a schematic diagram showing a configuration of a spontaneousemission source of a third embodiment in accordance with the presentinvention.

DESCRIPTION OF REFERENCE NUMERALS

1 Tm³⁺-doped fiber as a gain medium,

2 1.2 μm band pumping source

3 dichroic mirror

4 reflecting mirror,

5 2.3 μm band and 1.8 μm band band-pass filter

6 condenser lens

7 total reflection mirror

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described indetail in accordance with the foregoing distinctive features 1) and 2)of the present invention. (Description of Distinctive Feature 1 of thePresent Invention)

The fluorescence of an optical fiber doped with Tm at the 2.3 μm band isdue to the laser transition from the ³F₄ to ³H₅ level as shown inFIG. 1. To implement the laser application based on the transition, itis necessary to lengthen the residence time of Tm ions staying at the³F₄ level (that is, to prolong the life time of the fluorescence) toform the population inversion between the ³F₄ level and ³H₅ level. Toachieve this, it is considered important to reduce the nonradiativerelaxation of the Tm ions from the ³F₄ level to ³H₅ level. FIG. 3illustrates characteristics of the nonradiative relaxation rates of avarious types of glass. The nonradiative emission is caused bymulti-phonon relaxation from the ³F₄ level to ³H₅ level. The energydifference between the 3F₄ level and ³H₅ level is −4300 (1/cm). It isseen from FIG. 3 that tellurite glass, germanate hydroxide glass,fluoride glass, and chalcogenide glass have smaller nonradiativerelaxation rates than silica glass. Although not shown in FIG. 3,bismuth based glass and fluorophosphate glass have smaller nonradiativerelaxation rate than the silica glass. According to the knowledge, theinventors of the present invention fabricatedTm-doped optical fiberswith different glass hosts, and measured 2.3 μm band spontaneousemission spectra of 0.67 μm band excitation and 0.8 μm band excitation,the results of which are FIGS. 4, 5 and 6.

The specifications of the optical fibers used here are as follows:

Tm-doped fluoride fiber: Tm additive density is 2000 wt. ppm, relativerefractive index difference is 1.6%, cut off wavelength is 1.5 μm, andfiber length is 10 m;

Tm-doped tellurite fiber: Tm additive density is 2000 wt. ppm, relativerefractive index difference is 2.5%, cut off wavelength is 1.4 μm, andfiber length is 10 m;

Tm-doped chalcogenide glass fiber: Tm additive density 2000 wt. ppm,relative refractive index difference is 1.0%, cut off wavelength is 1.5μm, and fiber length is 5 m;

Tm-doped germanate hydroxide glass fiber: Tm additive density is 1500wt. ppm, relative refractive index difference is 1.1%, cut offwavelength is 1.3 μm, and fiber length is 10 m;

Tm-doped silica fiber: Tm additive density is 1500 wt. ppm, relativerefractive index difference is 1.8%, cut off wavelength is 1.2 μm, andfiber length is 10 m;

Tm-doped bismuth based glass fiber: Tm additive density is 1000 wt. ppm,relative refractive index difference is 2.5%, cut off wavelength is 1.43μm, and fiber length is 3 m;

Tm-doped fluorophosphate glass fiber: Tm additive density is 2500 wt.ppm, relative refractive index difference is 1.1%, cutoff wavelength is1.36 m, and fiber length is 3.5 m; and

Tm-doped phosphate glass fiber: Tm additive density is 1800 wt. ppm,relative refractive index difference is 1.55%, cut off wavelength is1.53 μm, and fiber length 2.9 m.

In addition, the pumping light intensity is 200 mW for the 0.67 μm band,and 150 mW for the 0.8 μm band.

The 2.3 μm band spontaneous emission was observed in the tellurite glassfiber, germanate hydroxide glass fiber, fluoride fiber, chalcogenideglass fiber, bismuth based glass fiber and fluorophosphate glass fiberwhich have smaller nonradiative relaxation rates than the silica glassas illustrated in FIGS. 4-6. On the other hand, the 2.3 μm bandspontaneous emission was not observed in the phosphate glass fiberhaving a greater nonradiative relaxation rate than the silica basedfiber or silica glass. This makes it clear that the 2.3 μm bandfluorescence is achieved by optical fibers using, as the host glass,various types of glass with the smaller nonradiative relaxation ratesthan the silica glass caused by the multi-phonon relaxation. Thus,according to the fluorescence, it is clearly seen that the laserapplication in the 2.3 μm wavelength region is possible by using theglass having the smaller nonradiative relaxation rate than the silicaglass caused by the multi-phonon relaxation as the Tm-doped glass host.

(Description of Distinctive Feature 2 of the Present Invention)

First, 2.3 μm band laser application (fiber laser, spontaneous emissionsource, and optical amplifier) according to the 1.2 μm band excitationto the Tm-doped fiber will be described. The 2.3 μm band laserapplication by this excitation is considered to be implemented byestablishing population inversion between the ³F₄ level and ³H₅ level byexciting thulium ions from the ³H₆ lowest energy level to the ³H₅ levelby 1.2 μm band excitation, followed by relaxation from that level to the³H₄ level through nonradiative process, by excitation from the ³H₄ levelto the ³F₂ level, and finally by relaxation of the thulium ions from the³F₂ level to the ³F₄ level through the nonradiative process.(Incidentally, as for the implementation of the 2.3 μm band laserapplication (fiber laser, spontaneous emission source and opticalamplifier) using the 1.2 μm band (1.2 μm band excitation) as theexcitation wavelength launched into the Tm-doped fiber, there have beenno reports up to now.)

FIG. 7 illustrates spontaneous emission spectra of the 1.2 μm bandexcitation Tm³⁺-doped fluoride fiber and Tm³⁺-doped tellurite fiber. Itis seen from FIG. 7 that spontaneous emission spectra of 2.3 μm bandcaused by the transition ³F₄ level→³H₅ level take place (although thefluorescence peaks are 2.05 μm, they are shifted by 1.2 μm band pumpinglight intensity). The fact that the 1.2 μm band pumping light intensitybrings about the fluorescence at the 2.3 μm band by the Tm-dopedfluoride fiber and Tm-doped tellurite glass fiber is new knowledgeobtained by the present inventors, which has not been known before. The2.3 μm band fluorescence cannot be observed with the Tm-doped silicabased fiber.

In addition, FIG. 8 illustrate spontaneous emission characteristics of a1.2 μm band excitation Tm³⁺-doped germanate hydroxide glass fiber,Tm³⁺-doped chalcogenide glass fiber, Tm³⁺-doped bismuth based glassfiber, and Tm³⁺-doped fluorophosphate glass fiber. The characteristicsare also new knowledge obtained by the present inventors just as thoseof the 1.2 μm band excitation Tm³⁺-doped fluoride fiber and Tm³⁺-dopedtellurite fiber. Besides, the inventors disclose that the 2.3 μm bandfluorescence is achieved by the 1.2 μm band excitation with the opticalfibers using glass with a smaller nonradiative relaxation rate than thesilica glass as the host glass, which is explained as the distinctivefeature 1) of the present invention. In other words, this makes it clearthat the 2.3 μm band laser application is possible by the 1.2 μm bandexcitation.

Incidentally, as for the Tm³⁺-doped fluoride fiber using the 1.2 μm bandexcitation, there have been no reports. However, 1.9 μm band laseroscillation by a Tm-Ho codoped fiber into which both Tm³⁺ and holmium(Ho) are doped is reported (see, non-patent document 3). The report,however, neither utilizes the laser transition of Tm³⁺ from the ³H₄ to³H₅ level, nor relates to the 2.3 μm band.

Next, a method will be described of achieving more reliable fibercharacteristics without degradation due to the photo darkening. FIG. 9illustrates a loss spectrum (solid curves) before 1.21 μm bandsemiconductor LD (laser diode) light of about 500 mW is launched into aTm-doped fluoride fiber (with an additive density of 2000 wt. ppm, afiber length of 20 m, and a relative refractive index difference of3.7%); and a loss spectrum (dash-dotted curves) 100 hours afterlaunching that light. As seen from FIG. 9, employing the 1.2 μm bandexcitation can suppress the photo darkening that increases the loss ofthe fluoride fiber itself, thereby enabling the highly reliable,practical 2.3 μm band laser application. (In FIG. 9, although thespectrum before launching the pumping light differs slightly from thespectrum after launching the 1.21 μm band pumping light, the differencecan be considered to be an error because of the measurement accuracy.)Furthermore, the following Table 1 shows the changes of the losses atthe wavelength 600 nm between the 1.047 μm band excitation and 1.21 μmband excitation of the Tm³⁺-doped tellurite fiber, Tm³⁺-doped germanatehydroxide glass fiber, Tm³⁺-doped chalcogenide glass fiber, Tm³⁺-dopedbismuth based glass fiber and Tm³⁺-doped fluorophosphate glass fiber(which were measured using fibers with the specifications shown in thedescription of the distinctive feature 1) of the present invention). Itis seen from the measurement results that the 1.2 μm band excitation isalso effective for increasing the reliability of other glass fibers ofthe fluoride fiber. TABLE 1 loss increase per loss increase per unitlength after unit length after 1.047 μm band 1.2 μm band excitation(500mW, excitation(500 mW, 56 hours) 100 hours) (wavelength 600 nm)(wavelength 600 nm) Fibers (dB/m) (dB/m) tellurite glass 0.81 <0.01fiber germanate 0.92 <0.01 hydroxide glass fiber chalcogenide glass 0.77<0.01 fiber bismuth based glass 0.71 <0.01 fiber fluorophosphate 0.85<0.01 glass fiber

The present invention will now be described in more detail withreference to the accompanying drawing. The embodiments in accordancewith the present invention disclosed below, however, are only examples,and do not impose any limitations on the scope of the present invention.

EMBODIMENT 1

In the first embodiment in accordance with the present invention,applications of the present invention to the 2.3 μm band and 1.8 μm bandfiber lasers will be described. FIG. 10 shows a configuration of thefirst embodiment in accordance with the present invention. In FIG. 10,the reference numeral 1 designates a Tm³⁺-doped fiber serving as a gainmedium; 2 designates a 1.2 μm band pumping source (consisting of asemiconductor laser with an oscillation wavelength of 1.21 μm, and amaximum output of 200 mW); 3 designates a dichroic mirror (that reflects1.2 μm band light and transmits 1.6-2.4 μm band light); 4 designates areflecting mirror (with the reflectance of 50% at 1.6-2.4 μm band, buttransmits 100% of the 1.2 μm band light); 5 designates a 2.3 μm band and1.8 μm band band-pass filter (with the transmission characteristics asillustrated in FIGS. 11A and 11B); 6 designates a condenser lens; and 7designates a total reflection mirror (with the reflectance of 95% ormore for 1.6-2.4 μm band light). As the Tm³⁺-doped fiber 1, thefollowing various types of the doped fibers were used one by one.

The specifications of the Tm³⁺-doped fiber 1 used are as follows:

Tm-doped fluoride fiber: Tm additive density is 2000 wt. ppm, relativerefractive index difference is 1.6%, cut off wavelength is 1.5 μm, andfiber length is 5 m;

Tm-doped tellurite fiber: Tm additive density is 2000 wt. ppm, relativerefractive index difference is 2.5%, cut off wavelength is 1.4 μm, andfiber length is 5 m;

Tm-doped chalcogenide glass fiber: Tm additive density is 2000 wt. ppm,relative refractive index difference is 1.0%, cut off wavelength is 1.5μm, and fiber length is 6 m;

Tm-doped germanate hydroxide glass fiber: Tm additive density is 1500wt. ppm, relative refractive index difference is 1.1%, cut offwavelength is 1.3 μm, and fiber length 5 m;

Tm-doped bismuth based glass fiber: Tm additive density is 1000 wt. ppm,relative refractive index difference is 2.5%, cut off wavelength is 1.43μm, and fiber length is 4.5 m; and

Tm-doped fluorophosphate glass fiber: Tm additive density is 2500 wt.ppm, relative refractive index difference is 1.1%, cutoff wavelength is1.36 μm, and fiber length is 5.5 m.

The 1.2 μm band pumping light intensity launched into each Tm³⁺-dopedfiber 1 was 50 mW, and when a 2.3 μm band band-pass filter (with atransmission central wavelength of 2.205 μm) was used as the band-passfilter 5, the following laser oscillations were achieved at 2.205 μmrespectively: 1.5 mW when the Tm-doped fluoride fiber was used; 2.2 mWwhen the Tm-doped tellurite fiber was used; 0.6 mW when the Tm-dopedchalcogenide glass fiber was used; 0.4 mW when the Tm-doped germanatehydroxide glass fiber was used; 1.3 mW when the Tm-doped bismuth basedglass fiber was used; and 1.1 mW when the Tm-doped fluorophosphate glassfiber was used.

Furthermore, when the 1.2 μm band pumping light intensity launched intoeach Tm-doped fiber 1 was 50 mW, and when a 1.8 μm band band-pass filter(with a transmission central wavelength of 1.801 μm) was used as theband-pass filter 5, the following laser oscillations were achieved at1.801 μm respectively: 2.4 mW when the Tm-doped fluoride fiber was used;3.2 mW when the Tm-doped tellurite fiber was used; 0.8 mW when theTm-doped chalcogenide glass fiber was used; 0.7 mW when the Tm-dopedgermanate hydroxide glass fiber was used; 1.9 mW when the Tm-dopedbismuth based glass fiber was used; and 1.4 mW when the Tm-dopedfluorophosphate glass fiber was used.

In addition, replacing the band-pass filter 5 by a tunable filterenables the laser oscillation at both the 2.3 μm band and 1.8 μm band.For example, using the Tm-doped fluoride fiber and the tunable filter itwas possible to achive the wavelength variable in the 1.75-2.21 μm band.

FIG. 12 illustrates the time stability of the output light intensity ofthe fiber laser using the Tm³⁺-doped fluoride fiber in the presentembodiment (2.205 μm laser initial output is 1.5 mW). FIG. 12 alsoillustrates the characteristics at 0.67 μm band excitation (the samelaser initial output as above). It was confirmed from these results thata highly reliable fiber laser application was possible by using the 1.2μm band excitation.

In addition, when using the Tm-doped tellurite fiber, Tm-dopedchalcogenide glass fiber, Tm-doped germanate hydroxide glass fiber,Tm-doped bismuth based glass fiber, and Tm-doped fluorophosphate glassfiber, variation in the laser output light intensity after 1000 houroperation were less than 10%, which made it clear that these glassfibers were also able to achieve the highly reliable fiber laserapplication by using the 1.2 μm band excitation.

EMBODIMENT 2

In the second embodiment in accordance with the present invention,application of the present invention to a 2.3 μm band optical fiberamplifier will be described. FIG. 13 shows a configuration of the secondembodiment in accordance with the present invention. In FIG. 13, thereference numeral 1 designates a Tm³⁺-doped fiber serving as a gainmedium; 2 designates a 1.2 μm band pumping source (consisting of asemiconductor laser with an oscillation wavelength of 1.21 μm, and amaximum output of 200 mW); 3 designates a dichroic mirror (that reflects1.2 μm band light and transmits 1.6-2.4 μm band light); and 6 designatesa condenser lens.

Using the following fibers as the gain medium 1 was able to implementthe following signal gains respectively:

Using the Tm-doped fluoride fiber (with Tm additive density of 2000 wt.ppm, relative refractive index difference of 1.6%, and fiber length of11 m) was able to achieve the signal gain of 8.3 dB for the 2.205 μmsignal light (when the 1.2 μm band pumping light intensity was 62 mW);

Using the Tm-doped tellurite fiber (with Tm additive density of 2000 wt.ppm, relative refractive index difference of 2.5%, and fiber length of 5m) was able to achieve the signal gain of 5.8 dB for the 2.205 μm signallight (when the 1.2 μm band pumping light intensity was 52 mW);

Using the Tm-doped chalcogenide glass fiber (with Tm additive density of2000 wt. ppm, relative refractive index difference of 1.0%, and fiberlength of 5 m) was able to achieve the signal gain of 3.8 dB for the2.205 μm signal light (when the 1.2 μm band pumping light intensity was75 mW);

Using the Tm-doped germanate hydroxide glass fiber (with Tm additivedensity of 1500 wt. ppm, relative refractive index difference of 1.0%,and fiber length of 6 m) was able to achieve the signal gain of 2.7 dBfor the 2.205 μm signal light (when the 1.2 μm band pumping lightintensity was 73 mW);

Using the Tm-doped bismuth based glass fiber (with Tm additive densityof 1000 wt. ppm, relative refractive index difference of 2.5%, and fiberlength of 5.5 m) was able to achieve the signal gain of 4.7 dB for the2.205 m signal light (when the 1.2 μm band pumping light intensity was55 mW); and

Using the Tm-doped fluorophosphate glass fiber (with Tm additive densityof 2500 wt. ppm, relative refractive index difference of 1.1%, and fiberlength of 4.3 m) was able to achieve the signal gain of 2.2 dB for the2.205 μm signal light (when the 1.2 μm band pumping light intensity was86 mW).

In addition, when using the foregoing various types of Tm-doped fibers,the signal gains could also be achieved at the 1.8 μb and under theforegoing excitation conditions: when using the Tm-doped fluoride fiber,the laser oscillation of 6.2 dB was achieved (at wavelength 1.805 μm);when using the Tm-doped tellurite fiber, the laser oscillation of 5.1 dBwas achieved (at wavelength 1.805 μm); when using the Tm-dopedchalcogenide glass fiber, the laser oscillation of 3.2 dB was achieved(at wavelength 1.805 μm); when using the Tm-doped germanate hydroxideglass fiber, the laser oscillation of 3.2 dB was achieved (at wavelength1.805 μm); when using the Tm-doped bismuth based glass fiber, the laseroscillation of 7.5 dB was achieved (at wavelength 1.805 μm); and whenusing the Tm-doped fluorophosphate glass fiber, the laser oscillation of2.8 dB was achieved (at wavelength 1.805 μm).

EMBODIMENT 3

In the third embodiment in accordance with the present invention,application of the present invention to a 2.3 μm band spontaneousemission source will be described. FIG. 14 shows a configuration of thethird embodiment in accordance with the present invention. In FIG. 14,the reference numeral 1 designates a Tm³⁺-doped fiber serving as a gainmedium; 2 designates a 1.2 μm band pumping source (consisting of asemiconductor laser with an oscillation wavelength of 1.21 μm, and amaximum output of 200 mW); 3 designates a dichroic mirror (that reflects1.2 μm band light and transmits 2.2 μm band light); and 6 designates acondenser lens.

The configuration of FIG. 14 enables the Tm³⁺-doped fluoride fiber,Tm³⁺-doped tellurite fiber, Tm³⁺-doped germanate hydroxide glass fiber,Tm³⁺-doped chalcogenide glass fiber, Tm³⁺-doped bismuth based glassfiber and Tm³⁺-doped fluorophosphate glass fiber to achieve thespontaneous emission characteristics as illustrated in FIGS. 7 and 8.The characteristics could implement the spontaneous emission sourceoperating at the 2.3 μm band. In addition, it is seen from FIGS. 7 and 8that the spontaneous emission can be used not only at the 2.3 μm band,but also at the 1.8 μm band.

OTHER EMBODIMENTS

Although the foregoing first to third embodiments employ thesemiconductor laser as the pumping source, other light sources such as a1.2 μm band fiber Raman laser are also applicable.

The present invention has been described by way of example of preferredembodiments. However, the embodiments in accordance with the presentinvention are not limited to the foregoing examples, and a varietymodifications such as replacement, changes, addition, increase ordecrease in the number, or the changes in the geometry of the componentsof the configuration are all included in the embodiments in accordancewith the present invention as long as they fall within the scope of theclaims.

INDUSTRIAL APPLICABILITY

Today, noninvasive blood glucose level sensing has hidden possibilitiesof a huge business, and many foreign and domestic medical inspectioninstrument developers compete fiercely to develop. In the noninvasiveblood glucose level sensing, the 2.3 μm band is one of the promisingglucose inspection wavelength bands, and this creates great demands fordeveloping the light sources operating at that wavelength band. As forthe light sources operating in the wavelength band, since thesemiconductor LDs are difficult to oscillate in this band, fiber lasersor spontaneous emission sources using a Tm-doped fluoride fiber havebeen developed. However, since they use 0.67 μm or 0.8 μm less than 1.05μm as the pumping light, the phenomenon that causes the photo darkeningoccurs, which presents a problem of reliability in that the 2.3 μm bandoutput light decreases with time, and becomes zero at last. The presentinvention can implement the highly reliable, practical fiber laser, ASEsource and optical fiber amplifier operating in the 2.3 μm band withoutdegradation in the fiber characteristics due to the photo darkening, andhence is very useful.

1. A fiber laser using as a gain medium an optical fiber that has a coreor a cladding doped with a rare-earth element having a laser transitionlevel, wherein said optical fiber is doped with at least thulium; andsaid fiber laser employs 1.2 μm band light or a pumping source forexciting the thulium from the lowest energy level ³H₆ to ³H₅ excitationlevel as a pumping source, and operates at least at 2.3 μm band.
 2. Thefiber laser as claimed in claim 1, wherein said optical fiber doped withthe thulium is a non-silica based fiber that uses glass having anonradiative relaxation rate which is caused by multi-phonon relaxationand is less than a nonradiative relaxation rate of silica glass as hostglass of the optical fiber.
 3. The fiber laser as claimed in claim 2,where said non-silica based fiber is one of a fluoride fiber, telluriteglass fiber, bismuth based glass fiber, fluorophosphate glass fiber,chalcogenide glass fiber, and germanate hydroxide glass fiber.
 4. Thefiber laser as claimed in claim 1, using laser transition at least from³F₄ to ³H₅ level.
 5. The fiber laser as claimed in claim 1, operating inboth 2.3 μm band and 1.8 μm band wavelength regions.
 6. The fiber laseras claimed in claim 1, using laser transition not only from ³F₄ to ³H₅level, but also from ³H₄ to ³H₅ level.
 7. A fiber laser using as a gainmedium an optical fiber that has a core or a cladding doped with arare-earth element having a laser transition level, said fiber laserhaving said optical fiber doped at least with thulium, and operating at2.3 μm band, wherein said fiber laser uses 0.67 μm band or 0.8 μm bandlight as a pumping source, and said optical fiber doped at least withthe thulium is a non-silica based fiber which uses, as host glass ofsaid optical fiber, glass having a nonradiative relaxation rate which iscaused by multi-phonon relaxation and is lower than a nonradiativerelaxation rate of silica glass.
 8. The fiber laser as claimed in claim7, wherein said optical fiber doped at least with the thulium is one ofa tellurite glass fiber, bismuth based glass fiber, fluorophosphateglass fiber, chalcogenide glass fiber, and germanate hydroxide glassfiber.
 9. The fiber laser as claimed in claim 7, using laser transitionfrom ³F₄ to ³H₅ level.
 10. A spontaneous emission source using as a gainmedium an optical fiber that has a core or a cladding doped with arare-earth element having a laser transition level, wherein said opticalfiber is doped with at least thulium; and said spontaneous emissionsource employs 1.2 μm band light or a pumping source for exciting thethulium from the lowest energy level ³H₆ to ³H₅ excitation level as apumping source, and operates at least at 2.3 μm band.
 11. Thespontaneous emission source as claimed in claim 10, wherein said opticalfiber doped with the thulium is a non-silica based fiber which uses, ashost glass of said optical fiber, glass having a nonradiative relaxationrate which is caused by multi-phonon relaxation and is lower than anonradiative relaxation rate of silica glass.
 12. The spontaneousemission source as claimed in claim 11, wherein said non-silica basedfiber is one of a fluoride fiber, tellurite glass fiber, bismuth basedglass fiber, fluorophosphate glass fiber, chalcogenide glass fiber, andgermanate hydroxide glass fiber.
 13. The spontaneous emission source asclaimed in claim 10, using laser transition at least from ³F₄ to ³H₅level.
 14. The spontaneous emission source as claimed in claim 10,operating in both 2.3 μm band and 1.8 μm band wavelength regions. 15.The spontaneous emission source as claimed in claim 10, using lasertransition not only from ³F₄ to ³H₅ level, but also from ³H₄ to ³H₅level.
 16. A spontaneous emission source using as a gain medium anoptical fiber that has a core or a cladding doped with a rare-earthelement having a laser transition level, said spontaneous emissionsource having said optical fiber doped at least with thulium, andoperating at 2.3 μm band, wherein said spontaneous emission source uses0.67 μm band or 0.8 μm band light as a pumping source, and said opticalfiber doped at least with the thulium is a non-silica based fiber whichuses, as host glass of said optical fiber, glass having a nonradiativerelaxation rate which is caused by multi-phonon relaxation and is lowerthan a nonradiative relaxation rate of silica glass.
 17. The spontaneousemission source as claimed in claim 16, wherein said optical fiber dopedat least with the thulium is one of a tellurite glass fiber, bismuthbased glass fiber, fluorophosphate glass fiber, chalcogenide glassfiber, and germanate hydroxide glass fiber.
 18. The spontaneous emissionsource as claimed in claim 17, using laser transition from ³F₄ to ³H₅level.
 19. An optical fiber amplifier using as a gain medium an opticalfiber that has a core or a cladding doped with a rare-earth elementhaving a laser transition level, wherein said optical fiber is dopedwith at least thulium; and said optical fiber amplifier employs 1.2 μmband light or a pumping source for exciting the thulium from the lowestenergy level ³H₆ to ³H₅ excitation level as a pumping source, andoperates at least at 2.3 μm band.
 20. The optical fiber amplifier asclaimed in claim 19, wherein said optical fiber doped with the thuliumis a non-silica based fiber that uses glass having a nonradiativerelaxation rate which is caused by multi-phonon relaxation and is lessthan a nonradiative relaxation rate of silica glass as host glass of theoptical fiber.
 21. The optical fiber amplifier as claimed in claim 20,where said non-silica based fiber is one of a fluoride fiber, telluriteglass fiber, bismuth based glass fiber, fluorophosphate glass fiber,chalcogenide glass fiber, and germanate hydroxide glass fiber.
 22. Theoptical fiber amplifier as claimed in claim 19, using laser transitionat least from ³F₄ to ³H₅ level.
 23. The optical fiber amplifier asclaimed in claim 19, operating in both 2.3 μm band and 1.8 μm bandwavelength regions.
 24. The optical fiber amplifier as claimed in claim19, using laser transition not only from ³F₄ to ³H₅ level, but also from³H₄ to ³H₅ level.
 25. An optical fiber amplifier using as a gain mediuman optical fiber that has a core or a cladding doped with a rare-earthelement having a laser transition level, said optical fiber amplifierhaving said optical fiber doped at least with thulium, and operating at2.3 μm band, wherein said optical fiber amplifier uses 0.67 μm band or0.8 μm band light as a pumping source, and said optical fiber doped atleast with the thulium is a non-silica based fiber which uses, as hostglass of said optical fiber, glass having a nonradiative relaxation ratewhich is caused by multi-phonon relaxation and is lower than anonradiative relaxation rate of silica glass.
 26. The optical fiberamplifier as claimed in claim 25, wherein said optical fiber doped atleast with the thulium is one of a tellurite glass fiber, bismuth basedglass fiber, fluorophosphate glass fiber, chalcogenide glass fiber, andgermanate hydroxide glass fiber.
 27. The optical fiber amplifier asclaimed in claim 25, using laser transition from ³F₄ to ³H₅ level.